The present invention relates to a semiconductor device and a method of forming the same, and more particularly to an improvement of current blocking layers in a semiconductor laser device and a method of forming the current blocking layers.
FIG. 1 is a schematic perspective view illustrative of a conventional semiconductor laser device having conventional current blocking layers. FIG. 2 is a diagram illustrative of an energy band gap profile across a double hetero-structure and a conventional current blocking layer as well as a cap layer overlying the conventional current blocking layer when no optical absorption into the current blocking layers appears. FIG. 3 is a diagram illustrative of an energy band gap profile across a double hetero-structure and a conventional current blocking layer as well as a cap layer overlying the conventional current blocking layer when an optical absorption into the current blocking layers appears.
An N-GaAs buffer layer 2 is provided on an n-GaAs substrate 1. An n-AlGaInP first cladding layer 3 is provided on the n-GaAs buffer layer 2. An active layer 4 is provided on the n-AlGaInP first cladding layer 3. The active layer 4 may comprise a multiple quantum well structure which comprises alternating laminations of GaInP well layers and AlGaInP barrier layers. A p-AlGaInP second cladding layer 5 is provided on the active layer 4. A p-GaInP etching stopper layer 6 is also provided on the p-AlGaInP second cladding layer 5. A p-AlGaInP stripe-shaped mesa structure 7 serving as a part of the cladding layer is provided on a selected region of the p-GaInP etching stopper layer 6. A p-GaInP hetero-buffer layer is provided on a top surface of the p-AlGaInP layer. GaAs current blocking layers 9 are provided on the p-GaInP etching stopper layer 6 so that the current blocking layers 9 are positioned in both sides of laminations of the p-AlGaInP stripe-shaped mesa structure 7 and the p-GaInP hetero-buffer layer 8. A p-GaAs cap layer 10 is provided over the p-GaInP hetero-buffer layer 8 and top surfaces of the current blocking layers 9. An n-electrode 11 is provided on a bottom surface of the n-GaAs substrate 1. A p-electrode 12 is also provided on a top surface of the p-GaAs cap layer 10. The above n-GaAs current blocking layers serve not only as a current blocking function for preventing a current leakage to outside of the mesa structure but also as an optical confinement of in a transverse direction due to its absorption of a light emitted from the active layer.
The above conventional laser device is engaged with a problem with an unintended turn-ON operation due to an absorption of a light into the current blocking layers. The current blocking layers are capable of absorbing a light emitted from the active layer, for which reason a part of the light L emitted from the active layer is absorbed into the current blocking layers. The absorption of the light into the current blocking layers generates electron-hole pairs whereby generated electrons are excited and lie on a conduction band of the current blocking layers whilst generated holes lie on a valence band of the current blocking layers. If a thickness of the current blocking layers is smaller than a diffusion length of holes as minority carriers in the current blocking layers, then the generated holes are diffused into adjacent layers, for example, the p-AlGaInP cladding layer 5 and the p-GaAs cap layer 10, whilst the generated electrons are accumulated in the current blocking layers. This causes a potential barrier drop of the current blocking layers whereby the balance band is made flat as illustrated in FIG. 3. As a result, a leakage of current of the holes as minority carriers toward the active layer appears. Namely, the current blocking layers are no longer capable of exhibiting current blocking functions. This mechanism was reported by S. Yamamoto et al. in Applied Physics Letters, vol. 40, pp. 372-374. 1982.
In order to avoid the above problem, it is required that the current blocking layers have a thickness which is sufficiently larger than the diffusion length of the minority carriers of the holes and also required that a dopant concentration of the current blocking layers is sufficiently risen. The increase in thickness of the current blocking layers also requires the increase in height of the mesa structure. The increase in height of the mesa structure narrows a width of a top portion of the mesa structure. The narrowing of the width of the top portion of the mesa structure increases a resistance to the current of the laser device. The increase in dopant concentration of the current blocking layers may raise a problem with a deterioration in surface homology of the current blocking layers and the cap layer overlying the current blocking layers.
In the above circumstances, it had been required to develop a novel laser device having improved current blocking layers free from the above problems.
Accordingly, it is an object of the present invention to provide a novel laser device having improved current blocking layers free from the above problems.
It is a further object of the present invention to provide a novel method of forming a laser device having improved current blocking layers free from the above problems.
It is a still further object of the present invention to provide an improved current blocking layer in a novel laser device free from the above problems.
It is yet a further object of the present invention to provide a novel method of forming an improved current blocking layer in a novel laser device free from the above problems.
The present invention provides a current blocking layer for a current confinement, the current blocking layer comprising a compound semiconductor crystalline, wherein the compound semiconductor crystalline has such a deviation from a stoichiometry in compositional ratio as introducing excess point defects into the compound semiconductor crystalline.
The present invention also provides a method of forming a current confinement structure in a semiconductor laser device. The method comprises the steps of: forming a cladding layer having a mesa structure and flat surface portions outside the mesa structure; and forming current blocking layers extending at least both on the flat surface portions of the mesa structure and on side faces of the mesa structure of the cladding layer, where each of the current blocking layers comprises a compound semiconductor crystalline, wherein the compound semiconductor crystalline is grown at a temperature of not higher than 0.4 times of a melting point of the compound semiconductor crystalline in an absolute temperature scale.
The above and other objects, features and advantages of the present invention will be apparent from the following descriptions.